update build/test condition on workflow and set v1.2.6

[sunya-ch]

This PR contains two commits:

1. correct the build condition to main branch and the test condition to all PRs
2. execute VERSION=1.2.6 make set_version

[minmzzhang]

Hi @sunya-ch , if we want to build on main instead of individual release, we don't need the following line in Makefile:

    @$(SED_CMD) -i '0,/branches:/s//branches:/; n; s/- v.*/- v$(VERSION)/' .github/workflows/*.yaml

[sunya-ch]

Hi @sunya-ch , if we want to build on main instead of individual release, we don't need the following line in Makefile:

    @$(SED_CMD) -i '0,/branches:/s//branches:/; n; s/- v.*/- v$(VERSION)/' .github/workflows/*.yaml

Thank you. I have removed the line.